The invention relates to limited rotation motors used to drive optical elements such as mirrors for the purpose of guiding light beams in scanners, and particularly relates to position transducers for such limited rotation motors.
Position transducers for limited rotation motors typically include a movable element that is attached to a rotor of a limited rotation motor, and a fixed element. For example, U.S. Pat. No. 3,970,979 discloses a capacitance position transducer that employs capacitive plates in determining the relative position of a rotor of a limited rotation motor. U.S. Pat. No. 4,864,295 also discloses a capacitive position transducer system. While capacitive position detection system as suitable for certain limited rotation motor applications, other applications require greater precision in position detection than that provided by capacitive position detection systems.
Position detection by optical sensing techniques have been disclosed in U.S. Pat. No. 5,235,180 using a pair of light emitting diodes, a rotating element that includes alternating diffusive and reflective surfaces attached to a rotor shaft, and a quad cell detector that receives reflected light from the rotating element's reflective surfaces.
U.S. Pat. No. 5,671,043 discloses a position detection system in which first and second light sources are directed toward an opaque element that is affixed to the rotor shaft. As the shaft rotates, the opaque element alternately blocks light from reaching four fixed position photocells that are positioned around the axis of rotation of the opaque element. U.S. Pat. No. 5,844,673 discloses a position detector that includes a fixed position axial light source that directs light toward a plurality of photodetectors that are positioned around the rotor shaft, and a butterfly-shaped blocking member that rotates with the rotor.
U.S. Pat. No. 6,921,893 discloses a position detection system for a scanning device that includes a pair of light sources on either side of a rotating shaft, a plurality of detectors that are fixed with respect to the shaft, and rotating light blocker that is interposed between the light sources and the detectors.
Such position detection systems, however, involve precise alignment and matching of light sources and photodetectors, and further require that the rotor shaft pass through the plane of the electronic circuitry, which increases manufacturing cost and alignment complexity. In certain applications it is also desired to provide a position detection system that has reduced drift, less sensitivity to mechanical misalignments and lower electrical noise, yet includes fewer parts, smaller size and lower manufacturing costs to produce.
There is a need, therefore, for an improved position detection system for a limited rotation motor system, and more particularly, there is a need for a position detection system for a limited rotation motor system that is efficient and economical to produce.